1. Field of the Invention
The present invention generally relates to semiconductor device fabrication. In particular, the present invention relates to a method for fabricating a thin-film transistor with a bottom-gate or dual-gate configuration.
2. Description of the Related Art
The thin-film transistor (TFT) is a semiconductor layer deposited on an insulating substrate which is thereafter made into a field-effect transistor. TFT devices are widely used in liquid crystal displays as the elements for driving or switching image pixels thereof, and, also they act as the active load within a static random access memory (SRAM) cell. In general, a TFT device with bottom-gate is a commonly used configuration. Referring to FIG. 1, a TFT device with bottom-gate configuration is shown in cross-sectional view. The bottom-gate TFT device comprises an insulating substrate 1, a gate electrode gate 10, a gate dielectric layer 11, a semiconductor layer 12, a pair of source/drain regions 13, and a dielectric layer 14. The numerals 15 represent a pair of contact windows for exposing the source/drain regions 13. Moreover, metal contacts 16 are connected to the source/drain regions 13 via the contact windows 15 to serve as source/drain electrodes.
The TFT device, as shown FIG. 1, is classified as an inverted staggered type, meaning that gate electrode 10 is first formed on the insulating substrate 1, and the source/drain regions 13 and gate electrode 10 are disposed at opposite sides of the semiconductor layer 12.
However, the main problem plaguing the bottom-gate TFT device is a self-aligned process is difficult to implement. Unlike a TFT device with a top-gate configuration, a gate electrode can serve as a mask while implanting ions to form a pair of source/drain regions, thus constituting a self-aligned process. Therefore, the bottom-gate TFT device not only suffers from greater parasitic capacitance, but also requires another photolithography procedure to define these source/drain regions. For the forgoing reasons, an improved self-aligned method for fabricating thin-film transistor is needed.